This is perfect timing, as after my App Engine talk in Prague, a nice gent came up to me and asked for just this. He wanted to process email in his application and didn’t think he could.

One shortcoming of purely HTTP-based webapps such as App Engine is
that they can’t receive email. I know that some people are wanting to
create App Engine apps that do this, so I put together a service to
facilitate this. It runs as a standard SMTP server, and sends all the
email it receives via HTTP POST to a user-designated URL. It’s free to
use (but not to abuse)